The present invention relates to an economical steel which is capable of preventing intergranular stress corrosion cracking that occurs when a steel is placed in contact with nitrogen oxides at temperatures below their dew point, rendering the steel unusable.
While the problem of air pollution has generally led to various attempts to ensure the complete combustion of gases at elevated temperatures, the combustion of gases at elevated temperatures has presented a number of new corrosion problems to which no consideration has heretofore been paid. In other words, when various gases are burnt at elevated temperatures, nitrogen oxides are eventually produced in very large amounts and these nitrogen oxides cause cracks in the steel parts of installations thus rendering them unfit for use and longer. Such cracks occur along the intergranular boundaries and one form of heretofore known cracking phenomenon similar to such cracks is stress corrosion cracking caused by nitrates. How such integranular stress corrosion cracking of steel due to nitrogen oxides occurs has not been clarified as yet. It is not conceivable, however, that gases such as NO, NO.sub.2 and NO.sub.3 per se have a direct effect on the causing of such cracks, but it is considered that condensation of such gases under the effects of temperature, pressure and the like, reaction of such gases with other substances present in the environment, or a complicated combination of such reactions results in the formation of NO.sub.3.sup.- ions and these NO.sub.3.sup.- ions are responsible for the occurrence of integranular cracking in the steel under applied or residual stresses. On the other hand, while there are also cases where alkaline solutions are frequently responsible for the intergranular stress corrosion cracking of a steel, the extent of this cracking is much less as compared with that of the cracking caused by nitrates. According, the safety of a metal against its intergranular stress corrosion cracking in alkaline solutions has heretofore been evaluated by the results of the tests in nitrates.
Further, in the above-mentioned nitrate corrosive environments, a passive thin film of magnetite is formed on the surface of steel and thus the general corrosion is not so severe and there is a tendency to use ordinary plain carbon steels in these environments. However, when such steels are used in these environments under stress which may be the residual stresss set up therein during welding, or working stress applied during operation, integranular stress corrosion cracking occurs which pevents accurate estimation of the life of the steel. Generally, while stainless steels show resistance to corrosion by nitrates or nitric acids and are thus well suited for use in these environments, it is very expensive and uneconomical to fabricate a large structure or the entire equipment of a stainless steel. For this reason, some steels which are inexpensive as compared with stainless steels, such as, 2% Cr - 0.8% Al steel and 0.4% Al steel have been developed and marketed for use in these environments. However, these steels must be subjected to heat treatments and contain alloying elements in large amounts. Moreover, in the manufacture of these steels, the addition of aluminum in a large amount tends to give rise to a poor fluidity, rough ingot skin and the like which result in an increase in the manufacturing costs, and therefore the manufacture of these steels is not preferred in the industry. Furthermore, the addition of aluminum to a steel tends to seriusly deteriorate the weldability and impact resistance properties of the steel and hence it is difficult to use them in a large welded structure. Thus, there has existed a need for the development of a steel containing aluminum in a relatively small amount. On the other hand, while it has been reported that subjecting the weld zones, of a structure made of ordinary plain carbon steel to a stress relieving annealing is effective in preventing cracking, this annealing results in a large distortion. Moreover, it is practically impossible to perform a stress relieving annealing on a large welded structure. This also involves difficult problems in respect of manufacture, delivery date and manufacturing costs.
Therefore, there exists the need for a steel which overcomes the foregoing difficulty relating to the weldability, impact resistance properties, workability and resistance to integranular stress corrosion cracking and which is as inexpensive as ordinary plain carbon steels.